1. Field of the Invention
The present invention relates to spark plugs of the wide discharge gap type having an insulating layer which is inserted between the electrodes of the spark plug. More particularly, the present invention relates to such spark plugs in which the insulating layer comprises a layer of an insulating material having a high dielectric constant and a layer of an insulating material which satisfies a desired heat resisting and high voltage resisting properties, thus improving the discharge characteristic and the ignition performance by utilizing the polarization effect of the insulating layer.
2. Description of the Prior Art
Recently, there has been an increasing tendency toward using leaner air-fuel mixtures and higher compression ratios from the standpoint of exhaust emission control and fuel consumption. In either case of using leaner mixtures and using higher compression ratios, spark plugs are required to ensure an improved ignition performance, reduced electrode wear and lower discharge voltage. While a wider discharge gap is generally effective in improving the ignition performance, there is a disadvantage that the discharge voltage increases with an increase in the combustion chamber pressure of an internal combustion engine and consequently it is impossible to widen the discharge gap beyond a certain limit.
Prior art spark plugs of the type effectively utilizing the polarization effect to decrease the discharge voltage and widening the discharge gap to improve the ignition performance are disclosed in Japanese Patent Applications Laid-open No. 50-20146 and No. 52-145647. The spark plug disclosed in the latter Patent Application is constructed so that an insulator insulating a center electrode from a plug shell is extended into the gap between the center electrode and the ground electrode such that the two electrodes face each other through the insulator, and the relative positions of the center electrode, the insulator and the ground electrode are determined by the relative dimensions of a creeping gap, a small gap and the thickness of the insulator. In this case, it is only necessary to widen the creeping gap for improving the ignition performance and the thickness of the insulator must be decreased as far as possible to decrease the discharge voltage. However, if the thickness is decreased, the insulator will break down through it and the insulator will be damaged, with the result that the discharge spark does not pass through the creeping surface and the discharge distance is decreased thus deteriorating the ignition performance. On the other hand, it may be possible to use an insulating material having a greater dielectric constant for the same thickness so as to further decrease the discharge voltage. In this case, while the use of a material having a greater dielectric constant can certainly decrease the discharge voltage, it is difficult to select a suitable material which satisfies both the resistance to heat and the resistance to high voltage required for plug insulators.